Abstract
Most parent compounds of iron-based superconductors (FeSCs) exhibit a tetragonal-to-orthorhombic lattice distortion below Ts associated with an electronic nematic phase that breaks the four-fold (C4) rotational symmetry of the underlying lattice, and then forms collinear antiferromagnetic (AF) below TN (TN ≤ Ts). Optimal superconductivity emerges upon suppression of the nematic and AF phases. FeSe, which also exhibits a nematic phase transition below Ts but becomes superconducting in the nematic phase without AF order, provides a unique platform to study the interplay amongst the nematic phase, AF order, and superconductivity. In this review, we focus on the experiments done on uniaxial pressure detwinned single crystals of FeSe and other FeSCs and highlight the importance of understanding the electronic and magnetic anisotropy in elucidating the nature of unconventional superconductivity.
Highlights
In unconventional superconductors such as heavy fermions, copper- and iron-based materials, the observation that superconductivity often emerges from their antiferromagnetic (AF) ordered parent compounds suggests that magnetism plays an important role in the mechanism of high-transition temperature superconductivity [1]
It is proposed that exotic state like Fulde–Ferrell– Larkin–Ovchinnikov (FFLO) state is realized in this compound [9, 10]
To study the intrinsic electronic and magnetic anisotropies, it is essential to detwin the sample (Figures 1D–F,J,K), which can be achieved by applying a uniaxial pressure through a mechanical clamp device that directly presses on the parallel edges of the crystal [4, 34, 37,38,39,40,41,42]
Summary
In unconventional superconductors such as heavy fermions, copper- and iron-based materials, the observation that superconductivity often emerges from their antiferromagnetic (AF) ordered parent compounds suggests that magnetism plays an important role in the mechanism of high-transition temperature (high-Tc) superconductivity [1]. Since the tetragonal-to-orthorhombic structural transition for FeSCs occurs below room temperature (Ts < 295 K), the system forms 90◦ rotated twinned domains below Ts, making it impossible for a bulk probe to determine the intrinsic electronic and magnetic properties of the individual domains and the associated nematic phase. To alleviate this technical difficulty, mechanical detwin devices were developed first for the BaFe2As2 compounds [4, 5], and later adapted for other material families. Given the contrasting behavior between Ba(Fe1−xCox)2As2 and FeSe1−xSx, it will be interesting to study the relationship between nematic phase and superconductivity in these two families of materials
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